The demands for water in oil sands operations are high and therefore most operations must rely on recycling process water. However, during oil sands processing, a significant amount of dissolved inorganic (e.g., salts) and organic (e.g., carboxylic acids, hydrocarbon) constituents are released into process waters. Recycling of the oil sands process-affected water (OSPW) only serves to increase the levels of dissolved inorganic and organic content. Currently no OSPW is released from the operations.
In order to meet water quality criteria for release, it is necessary to treat the OSPW to reduce the dissolved organics, such as naphthenic acids and other hydrocarbons. Naphthenic acids have been demonstrated to be toxic to aquatic biota (Alberta Environment Protection. 1996. Naphthenic acids background information discussion report. Edmonton, Alberta, Alberta Environment, Environmental Assessment Division). Thus, the concentration of naphthenic acids present in OSPW must be reduced to levels that are not detrimental to the biological community of a receiving aquatic system. Removal of naphthenic acids may be accomplished with either natural bioremediation or treatment methods to remove them from the OSPW.
Naphthenic acids (NAs) are natural constituents in many petroleum sources, including bitumen in the oil sands of Northern Alberta, Canada. NAs are complex mixtures of predominately low molecular weight (<500 amu), fully saturated alkyl-substituted acyclic and cycloaliphatic (one to more than six rings) carboxylic acids (Brient, J. A., Wessner, P. J., and Doyle, M. N. 1995. Naphthenic acids. In Encyclopedia of Chemical Technology, 4th ed.; Kroschwitz, J. I., Ed.; John Wiley & Sons: New York, 1995; Vol. 16, pp 1017-1029). They are described by the general empirical formula CnH2n+ZO2, where n indicates the carbon number and Z is zero or a negative, even integer that specifies the hydrogen deficiency resulting from ring formation (i.e. Z=−2 indicates 1-ring, Z=−4, 2-rings etc.). While some of naphthenic acids will biodegrade rapidly, a fraction of the naphthenic acids associated with the OSPW have been shown to be more recalcitrant (Scott, A. C., M. D. MacKinnon, and P. M. Fedorak. 2005. Naphthenic acids in Athabasca oil sands tailings waters are less biodegradable than commercial naphthenic acids. ES&T 39: 8388-8394). In order to facilitate release of OSPW, it is desirable to find options for more rapid removal of NAs from OSPW that is effective, targeted to the dissolved organics and economically viable.
In surface oil sands mining operations for recovery of bitumen, also referred to as open-pit oil sands operations, hot or warm water, to which a process aid, such as caustic (NaOH) may be added, is mixed with the oil sand ore (about 1.5-2 m3 of water per barrel of oil extracted) in order to separate the bitumen from the oil sand. The resulting oil sand slurry goes through a series of separators to produce lean bitumen froth. The tailings stream produced during bitumen extraction, which comprises water, sand and un-recovered bitumen, is transported to settling basins, where the solids settle by gravity, and the resulting release water (OSPW) is recycled for reuse in the extraction process. Also included as recycle or release water is seepage water from sand structures containing settling basins. It is during this extraction process that leaching of both inorganic and organic constituents will occur.
Bitumen can also be recovered from oil sands in situ (in the geological formation) using the Steam Assisted Gravity Drainage process (the “SAGD” process). SAGD requires the generation of large amounts of steam in steam generators, which steam is injected via injection wells to fluidize the bitumen for recovery. A bitumen/water mixture results and the mixture is pumped to the surface where the bitumen is separated from the water. The produced water stream (oil sands process-affected water) is then reused to produce more steam for extraction. As in surface mining operations, the produced water stream contains dissolved organics that need to be removed. The produced water in SAGD must be treated to meet requirements for once-through steam generators, and the retentate from this preparation will contain elevated NAs.
Bitumen produced from either surface mining operations or SAGD can be further upgraded by thermal cracking using either a delayed or fluid coker reactor, as are known in the art, to take the highly viscous bitumen (API gravity of about 8°) to a less viscous hydrocarbon product (API gravity of about 30°). During coking, an excess amount of petroleum coke is produced, which excess coke is currently disposed of as waste product. Thus, petroleum coke produced from coking operations is a readily available commodity.
There is a need for an effective, selective and economical water treatment process for the OSPW produced during bitumen oil extraction processes so that the water can be reused in the operation or released into the environment.